Modern electrical and electronic equipment using alternating current often requires a reliable energy source producing a constant frequency output. For example, in marine and other mobile applications a reliable constant-frequency power source is needed by sensitive electronic equipment, such as navigational equipment and other electrical devices.
The primary power source for propulsion on mobile systems, typically a diesel or gas-powered engine, is operated over a wide range of speeds as required to provide the desired motive force. The variable operating speed of the primary power engine has therefore been considered ill suited for driving the onboard electric power generator(s). Commercial and/or larger recreational watercraft frequently use on-board auxiliary diesel engines or the like that are operated at a constant speed to drive the electric generators for producing the necessary electric power for electrical and electronic equipment. The auxiliary engine is independent of the main engine, and therefore the system can provide electric power to the vessel even when the main engine is not operating. However, a major disadvantage of such conventional systems is that they require one or more auxiliary engines, and result in increased fuel consumption and related costs. Such systems are costly to purchase, install, maintain, and operate.
Alternatively, it is known that electric power may be generated using a shaft-driven generator powered directly by the vessel's main engine. A direct-drive system can produce electric power very economically and with relatively low additional effluent of hydrocarbon to the environment. However, to produce a stable electric power output a direct-drive electric generator system requires operation of the main engine for power, and at a constant speed, and is therefore only applicable to a few types of vessels.
Previously disclosed systems that attempt to overcome these problems generally provide a hydraulic drive with a hydrostatic transmission that variably connects a power generator to the vessel main engine. See, for example, U.S. Pat. No. 7,485,979 to Staalesen, which is hereby incorporated by reference in its entirety. Another system that provides a hydraulic drive that variably connects a power generator to a vessel main engine is described in U.S. patent application Ser. No. 09/801,049, also to Staalesen.
The system described in these prior art references drives a generator from the main engine via a variable displacement hydraulic pump that is fluidly connected to a constant volume hydraulic motor. A hydraulic pump controller is used to control the pump displacement, relying on the frequency deviation on the generator output for controlling the hydraulic pump. The system described in the Staalesen patent drives a generator from the main engine with a variable displacement hydraulic pump, wherein the controller uses a pump speed input signal and a power frequency input signal to control a variable hydraulic pump, such that the fluid flow from the variable displacement hydraulic pump is substantially constant.
However, it is desirable to improve on the stability of the power generator output to provide a more consistent output power supply to the ship's onboard electronic systems. The present disclosure describes a power system with improved output power stability.